The present invention relates to a capacitor, and more particularly to a solid electrolytic capacitor using an electrically conductive polymer and a method of forming the same.
The solid electrolytic capacitor has an anode, a dielectric film on surfaces of the anode, and a cathode on an outer surface of the dielectric film, so that the cathode is electrically separated by the dielectric film from the anode. The dielectric film may comprise an oxide film. The anode may comprise a porous form of a valve action metal such as tantalum, niobium or aluminum. The cathode may have a part made of a solid electrolyte which is in contact with an entire surface of the dielectric film. The solid electrolyte serves as an electrical connection between an electrode lead and the entire surface of the dielectric film. In view of a possible reduction in resistance of the capacitor, it is, of course, preferable that the solid electrolyte has a possible high electrical conductivity. It is further required that the solid electrolyte is capable of suppressing short circuit current due to defects of the dielectric film. The solid electrolyte is capable of exhibiting a transition into an insulator upon a heat generation due to the short circuit current. Therefore, the solid electrolyte is required to have both the possible high electrical conductivity and the transition capability into the insulator upon a heat generation due to the short circuit current, for which reason manganese dioxide or TCNQ complex have been used as the solid electrolyte. Particularly when the solid electrolytic capacitor is mounted on a printed circuit board, the solid electrolytic capacitor is then subjected to a heat at 240-260xc2x0 C. In this case, manganese dioxide has been used because manganese dioxide is thermally stable at a high temperature of at least 240xc2x0 C.
Consequently, the solid electrolyte for the solid electrolytic capacitor to be mounted on the printed circuit board is required to have the high electrical conductivity, the transition capability into the insulator upon the heat generation due to the short circuit current, and the thermal stability at the high temperature of at least 240xc2x0 C.
Manganese dioxide have the required transition capability and thermal stability but insufficient electrical conductivity of about 0.1 S/cm. Advanced solid electrolytic capacitors are required to have much higher electrical conductivity of 10-100 S/cm.
In the above recent circumstances, available solid electrolyte satisfying the above three requirements of transition capability, high thermal stability and high electrical conductivity is specified electrically conductive polymers, for example, polypyrrole, polythiophene, and polyaniline. Developments of such solid electrolytic capacitor using electrically conductive polymers as the solid electrolyte have been active. The solid electrolytic capacitor using polypyrrole as the solid electrolyte has already been commercialized.
In Japanese laid-open patent publication No 2-15611, it is disclosed that thiophene derivatives are polymerized with ferric compound to form polythiophene to be used as the solid electrolyte for the solid electrolytic capacitor. This polymer of thiophene derivatives is superior than a polymer of pyrrole derivatives in smaller drop of electrical conductivity in a high temperature atmosphere.
In Japanese laid-open patent publication No. 5-152169, it is disclosed that immediately after the dielectric film coating the anode has been dipped into an oxidizing agent solution with a solvent almost entirely consisting of water, then another solution including pyrrole monomers or thiophene monomers for causing polymerization reaction thereof so as to form a polypyrrole film, or a polythiophene film as the solid electrolyte. These polymers of pyrrole and thiophene are also superior in smaller drop of electrical conductivity in a high temperature atmosphere.
The above electrically conductive polymer film as the solid electrolyte coats the dielectric film which further coats the valve action metal anode. The above conventional method of forming the electrically conductive polymer film as the solid electrolyte on the dielectric film results in a formation of a thin electrically conductive polymer film that is incapable of withstanding mechanical stresses applied by expansion and shrinkage of an armored resin material which coats the electrically conductive polymer film. Particularly in the vicinity of a projecting anode lead, the electrically conductive polymer film is mechanically weak, for which reason the electrically conductive polymer film is required to have an increased strength against the mechanical stresses applied by expansion and shrinkage of the armored resin material.
Even if the number of polymerization is increased to increase the thickness of the electrically conductive polymer film as the solid electrolyte for improvement in capability of withstanding mechanical stresses applied by expansion and shrinkage of the armored resin material, antinomy problem is raised with increase in equivalent series resistance of the solid electrolytic capacitor, because the increase in thickness of the film laminated over the entire surface of the valve action metal anode results in increase in the equivalent series resistance of the solid electrolytic capacitor. Notwithstanding, it is extremely important for the advanced solid electrolytic capacitor to realize a possible reduction in the equivalent series resistance.
Consequently, the above conventional method of forming the electrically conductive polymer film as the solid electrolyte is engaged with the problem in antinomy to the effect that the increase in thickness of the electrically conductive polymer film for the purpose of improvement in capability of withstanding mechanical stresses applied by expansion and shrinkage of the armored resin material would result in increase in the equivalent series resistance, whilst the decrease in thickness of the electrically conductive polymer film for the purpose of reducing the equivalent series resistance would result in decrease in capability of withstanding mechanical stresses applied by expansion and shrinkage of the armored resin material.
In the above circumstances, it had been required to develop a novel solid electrolytic capacitor free from the above problems and a method of forming the same.
Accordingly, it is an object of the present invention to provide a novel solid electrolytic capacitor free from the above problems.
It is a further object of the present invention to provide a novel solid electrolytic capacitor having a possible high electrical conductivity.
It is a still further object of the present invention to provide a novel solid electrolytic capacitor having a capability of exhibiting a transition into an insulator upon a heat generation by a short circuit current due to defects of a dielectric film for suppressing the short circuit current.
It is yet a further object of the present invention to provide a novel solid electrolytic capacitor having a possible high thermal stability at a temperature of at least 240xc2x0 C.
It is a yet a further more object of the present invention to provide a novel solid electrolytic capacitor, wherein the electrically conductive polymer film has a sufficient thickness for withstanding mechanical stresses applied by expansion and shrinkage of an armored resin material, particularly in the vicinity of a projecting anode lead.
It is still more object of the present invention to provide a novel solid electrolytic capacitor having a reduced equivalent series resistance.
It is moreover object of the present invention to provide a novel method of forming an electrically conductive polymer film as a solid electrolyte for a solid electrolytic capacitor free from the above problems.
It is another object of the present invention to provide a novel method of forming an electrically conductive polymer film as a solid electrolyte for a solid electrolytic capacitor having a possible high electrical conductivity.
It is still another object of the present invention to provide a novel method of forming an electrically conductive polymer film as a solid electrolyte for a solid electrolytic capacitor having a capability of exhibiting a transition into an insulator upon a heat generation by a short circuit current due to defects of a dielectric film for suppressing the short circuit current.
It is yet another object of the present invention to provide a novel method of forming an electrically conductive polymer film as a solid electrolyte for a solid electrolytic capacitor having a possible high thermal stability at a temperature of at least 240xc2x0 C.
It is further another object of the present invention to provide a novel method of forming an electrically conductive polymer film as a solid electrolyte for a solid electrolytic capacitor, wherein the electrically conductive polymer film has a sufficient thickness for withstanding mechanical stresses applied by expansion and shrinkage of an armored resin material, particularly in the vicinity of a projecting anode lead.
It is an additional object of the present invention to provide a novel method of forming an electrically conductive polymer film as a solid electrolyte for a solid electrolytic capacitor having a reduced equivalent series resistance.
The present invention provides an electrically conductive polymer layer of a solid electrolytic capacitor, the electrically conductive polymer layer coating a dielectric layer coating an anode with an anode lead which projects outwardly from the anode through the dielectric layer, and the electrically conductive polymer layer being separated by the dielectric layer from the anode lead, wherein the electrically conductive polymer layer has a thicker portion than a remaining portion thereof and the thicker portion extends at least in the vicinity of the anode lead, and the thicker portion is bounded with the remaining portion so that a sloped-boundary loop between the thicker portion and the remaining portion extends to be parallel to a flat plane vertical to a direction along which the anode lead projects through the dielectric layer, whereby the thicker portion continuously extends closer to the anode lead than the remaining portion.
The another present invention also provides a method of forming an electrically conductive polymer layer on a surface of a dielectric layer coating an anode with an anode lead projecting from the anode through the dielectric layer. The method comprises the steps of: dipping the anode coated with the dielectric layer into a first solution including an oxidizing agent and having a first specific gravity; and subsequently dipping the anode coated with the dielectric layer which still remains wetted with a thin film of the first solution into a second solution including monomers of an electrically conductive polymer and being insoluble with the first solution and further having a second specific gravity larger than the first specific gravity in such a direction that an anode-lead formation position is positioned upwardly, so that the thin film of the first solution is partially floated to move upwardly so as to thickly coat an upper portion of the dielectric layer, whereby a polymerization reaction of the monomers is predominantly caused on the upper portion of the dielectric layer, thereby to form an electrically conductive polymer layer having a thicker portion than a remaining portion thereof, wherein the thicker portion extends on the upper portion of the dielectric layer. The boundary between the thicker portion and the remaining portion is not step-shaped but is slope-shaped. The boundary forms a sloped-boundary loop which extends to be parallel to a flat plane vertical to a direction that an anode-lead formation position is positioned upwardly, whereby the thicker portion continuously extends in a closer to the anode-lead formation position than the remaining portion.
The still another present invention also provides a method of forming an electrically conductive polymer layer on a surface of a dielectric layer coating an anode with an anode lead projecting from the anode through the dielectric layer. The method comprises the steps of: dipping the anode coated with the dielectric layer into a first solution including an oxidizing agent and having a first specific gravity; drying a surface of the dielectric layer coating the anode to form a dried thin film of the oxidizing agent which coats the surface of the dielectric layer; dipping the anode coated with the dielectric layer coated with the dried thin film of the oxidizing agent into a pure water to form a wet tin film containing the oxidizing agent; and subsequently dipping the anode coated with the dielectric layer coated with the wet thin film into a second solution including monomers of an electrically conductive polymer and being insoluble with the first solution and further having a second specific gravity larger than the first specific gravity in such a direction that the anode-lead formation position is positioned upwardly, so that the wet thin film is partially floated to move upwardly so as to thickly coat an upper portion of the dielectric layer, whereby a polymerization reaction of the monomers is predominantly caused on the upper portion of the dielectric layer, thereby to form an electrically conductive polymer layer having a thicker portion than a remaining portion thereof, wherein the thicker portion extends on the upper portion of the dielectric layer. The boundary between the thicker portion and the remaining portion is not step-shaped but is slope-shaped. The boundary forms a sloped-boundary loop which extends to be parallel to a flat plane vertical to a direction that the anode-lead formation position is positioned upwardly, whereby the thicker portion continuously extends in a closer to the anode-lead formation position than the remaining portion.
The above and other objects, features and advantages of the present invention will be apparent from the following descriptions.